Non-aqueous secondary battery having negative electrode including graphite powder

ABSTRACT

Objects of the present invention is to provide a carbon material having a superior reversibility in lithium intercalation-deintercalation reaction, and a non-aqueous secondary battery using the carbon material as an active material for a negative electrode, which has a high energy density and an excellent rapid charging and discharging characteristics.  
     Graphite powder having a maximum particle diameter of less than 100 μm and an existing fraction of rhombohedral structure in the crystalline structure of less than 20% is used as an active material for the negative electrode of the non-aqueous secondary battery. The graphite powder can be obtained by pulverizing raw graphite with a jet mill, and subsequently treating the powder at a temperature equal to or higher than 900° C.

[0001] This application is a Divisional application of application Ser.No. 09/473,300, filed Dec. 28, 1999, which is a Divisional applicationof application Ser. No. 08/630,501, filed Apr. 10, 1996.

BACKGROUND OF THE INVENTION

[0002] The present invention relates to a carbon material whichintercalates into or deintercalates from lithium, and to a method formanufacturing the same. In particular, the present invention relates toa lithium secondary battery, which uses carbon material as a negativeelectrode active material, having a high energy density and a long life.The lithium battery is suitable for use in portable apparatus, electricautomobiles, power storage, etc.

[0003] The Lithium secondary battery using lithium metal for thenegative electrode has some problems relating to safety. For example,lithium easily deposits like dendrite on the lithium metal negativeelectrode during repeated charging and discharging of the battery, andif the dendritic lithium grows to a positive electrode, an internalshort circuit will be caused between the positive electrode and thenegative electrode.

[0004] Therefore, a carbon material has been proposed as the negativeelectrode active material in place of lithium metal. Charge anddischarge reactions involve lithium ion intercalation into the carbonmaterial and deintercalation from the carbon material, and so lithium ishardly deposited like dendrite. As for the carbon material, graphite isdisclosed in JP-B-62-23433 (1987).

[0005] The graphite disclosed in JP-B-62-23433 (1987) forms anintercalation compound with lithium, because of intercalation ordeintercalation of lithium. Thus, graphite is used as a material for thenegative electrode of the lithium secondary battery. In order to usegraphite as the negative active material, it is necessary to pulverizethe graphite to increase the surface area of the active material, whichconstitutes a charge and discharge reaction field, so as to allow thecharging and discharging reactions to proceed smoothly. Desirably, it isnecessary to pulverize the graphite to powder having a particle diameterequal to or less than 100 μm. However, as is apparent from the fact thatgraphite is used as a lubricating material, the graphite easilytransfers its layers. Therefore, its crystal structure is changed by thepulverizing process, and formation of the lithium intercalated compoundmight be influenced by undesirable effects of the pulverizing process.Accordingly, the graphite after the pulverizing process has a great dealof crystalline structural defects. In a case when graphite is used as anactive material for the negative electrode of the lithium secondarybattery, a disadvantage results in that a large capacity can not beobtained. Furthermore, preferable performances of rapid charging anddischarging are not obtained because the lithiumintercalation-deintercalation reaction is disturbed by the abovedefects.

SUMMARY OF THE INVENTION

[0006] The object of the present invention is to soive the aboveproblems, to provide a carbon material having a large lithiumintercalation-deintercalation capacity and a method for manufacturingthe same, and to provide a non-aqueous secondary battery which has alarge capacity and is superior in its rapid charging and dischargingcharacteristics using the above-mentioned materials.

[0007] The crystalline structure of the graphite powder relating to thepresent invention has a feature that an existing fraction of therhombohedral structure in the crystalline structure of the graphite issmall (equal to or less than 20%). Another feature is that an existingfraction of the hexagonal structure is great (at least 80%). The aboveexisting fractions of the rhombohedral structure and the hexagonalstructure can be determined by analyzing the intensity ratio of thepeaks in X-ray diffraction of the material.

[0008] The graphite powder relating to the present invention ismanufactured by a method comprising the steps of graphitizing treatment(heating at least 2000° C.) of raw material such as oil cokes and coalcokes, pulverizing the graphitized raw material to powder, sieving thepowder for obtaining the maximum particle diameter equal to or less than100 μm, heating the powder to at least 900° C. as a heat treatment, andfurther heating the powder to at least 2700° C. for eliminatingimpurities such as Si. For instance, when the powder is heated to atleast 2700° C., Si, which is a main component of the impurities, can bereduced to less than 10 ppm. The heat treatment of the powder foreliminating impurities can be omitted depending on the content of theimpurities in the raw material. In the pulverizing process, variousconventional pulverizers can be used. However, a jet mill is preferable,because pulverization with the jet mill generates the minimumdestruction of the graphite crystalline structure in the raw material.

[0009] Furthermore, the graphite powder relating to the presentinvention can be obtained by immersing into an acidic solutioncontaining at least one compound selected from a group consisting ofsulfuric acid, nitric acid, perchloric acid, phosphoric acid, andfluoric acid as an immersing treatment, after pulverizing the rawgraphite to obtain graphite powder having a particle diameter equal toor less than 100 μm, subsequently washing with water, neutralizing, anddrying.

[0010] The non-aqueous secondary battery for achieving the object of thepresent invention can be manufactured by using the graphite powderrelating to the present invention as the negative electrode activematerial, and the positive electrode is desirably composed of a materialcomprising a compound expressed by a chemical formula of Li_(x)MO₂(where; X is in a range from zero to 1, and M is at least any one ofchemical elements selected from a group of Co, Ni, Mn and Fe), orLiMn₂O₄, that is a lithium transient metal complex oxide.

[0011] The active materials for the battery are generally used in theform of a powder in order to facilitate the charging and dischargingreaction by increasing the surface area of the active material, whichconstitutes a reaction field of the charging and discharging reaction.Therefore, the smaller the particle size of the powder is, the more willperformance of the battery be improved. Furthermore, when the electrodeis manufactured by applying an agent mixed with the active material anda binding agent to a current collector, the particle diameter of theactive material is desirably equal to or less than 100 μm in view ofapplicability and maintaining preciseness of thickness of the electrode.

[0012] As for the negative electrode active material for the lithiumsecondary battery, natural graphite, artificial graphite, and others aredisclosed. However, for the above described reason, it is necessary topulverize these materials. Therefore, in the pulverizing process,various graphite powders having a diameter equal to or less than 100 μmwere prepared with various pulverizing methods using a ball mill, a jetmill, a colloidal mill and other apparatus, for various times. And, thelithium intercalation-deintercalation capacity of the various graphitepowders were determined for determining a superior material for thenegative electrode material of the lithium secondary battery.

[0013] However, the graphite powder obtained by the above method hadlithium intercalation-deintercalation amounts per weight in a range of200-250 mAh/g, and their capacities as the material for the negativeelectrode of the lithium secondary battery were not enough.

[0014] In order to investigate, the reason for the small capacity,crystalline structures of the above various graphite samples weredetermined by an X-ray diffraction method. FIG. 1 indicates an exampleof the results. Four peaks can be observed in a range of the diffractionangle (2θ, θ: Bragg angle) from 40 degrees to 50 degrees in the X-raydiffraction pattern. The peaks at approximately 42.3 degrees and 44.4degrees are diffraction patterns of the (100) plane and the (101) planeof hexagonal structure of the graphite, respectively. The peaks atapproximately 43.3 degrees and 46.0 degrees are diffraction patterns ofthe (101) plane and the (102) plane of the rhombohedral structure of thegraphite, respectively. As explained above, it was apparent that therewere two kinds of crystalline structure in the pulverized graphite.

[0015] Further, the existing fraction (X) of the rhombohedral structurein the graphite powder was calculated by the following equation(Equation 1) based on the data of the observed peak intensity (P₁) ofthe (100) plane of the hexagonal structure, the observed peak intensity(P₂) of the (101) plane of the rhombohedral structure, and a theoreticalrelationship of the intensity ratio in the X-ray pattern of graphite. Asa result, it was revealed that the graphite having the rhombohedralstructure was contained by approximately 30% in all the graphitepulverized equal to or less than 100 μm in particle diameter.

X=3P ₂/(11P ₁+3P ₂)  (Equation 1)

[0016] Similarly, the existing fraction (X) of the rhombohedralstructure of the graphite powder was verified by the relationship of theobserved peak intensity (P₁) of the (100) plane of the hexagonalstructure, the observed peak intensity (P₃) of the (102) plane of therhombohedral structure, and the theoretical relationship of theintensity ratio in the X-ray pattern of the graphite. In this case, thefollowing equation 2 was used instead of the equation 1. As a result, itwas confirmed that graphite having the rhombohedral structure wascontained by approximately 30% in all the graphite pulverized equal toor less than 100 μm in particle diameter.

X=P ₃/(3P ₁ +P ₃)  (Equation 2)

[0017] The reason for existence of the two kinds of crystallinestructure is assumed to be that the graphite itself has a lubricatingproperty, and the original graphite having a hexagonal structuretransforms to graphite having rhombohedral structure by the pulverizingprocess with strong shocks. Graphite powder of a few microns in particlediameter obtained by further continued pulverization had a significantlybroadened X-ray diffraction peak (P₄) at the (101) plane of thehexagonal structure, and it was revealed that the content of amorphouscarbon in the graphite was increased because the half band width of thepeak was increased. Accordingly, the reason for the small lithiumintercalation-deintercalation capacity of the conventional graphitepowder can be assumed to be due to the fact that the crystallinestructure of the graphite has been transformed to the rhombohedralstructure and has generated the amorphous carbon, with the result thatthe lithium intercalation-deintercalation reaction is disturbed by therhombohedral structure and the amorphous carbon.

[0018] Analysis of the impurities of the graphite powder revealed thatimpurities such as Si, Fe, and others were present in an amount morethan 1000 ppm. Naturally, in addition to the impurities contained in theraw material, impurities from a processing apparatus, such as a ballmill, a jet mill, and the like, can be mixed into the graphite duringthe pulverizing process. Therefore, the influence of the aboveimpurities can be assumed as another reason for the small capacity, inaddition to the above formation of the rhombohedral structure andamorphous carbon.

[0019] In accordance with the present invention, a graphite powderhaving a particle diameter equal to or less than 100 μm, wherein thecontent of the above described rhombohedral structure is less than 30%and the content of the amorphous carbon is small, has been developed.Additionally, the content of Si in particular, which is the maincomponent of the impurities in the graphite powder, has been decreasedto an amount equal to or less than 10 ppm. Therefore, extremely highpurity is one of the features of the graphite relating to the presentinvention. The particle diameter equal to or less than 100 μm isdetermined with an intention to use the graphite for a battery, asdescribed previously. Therefore, when the graphite of the presentinvention is used for other purposes, the particle diameter of thegraphite is not necessarily restricted to a size equal to or less than100 μm.

[0020] Hereinafter, details of the graphite powder relating to thepresent invention, and the method for manufacturing the same will beexplained.

[0021] Two methods (manufacturing method 1 and manufacturing method 2)for obtaining graphite having a small fraction of the rhombohedralstructure are disclosed.

[0022] (Manufacturing Method 1)

[0023] As for raw material (raw graphite) for the graphite powder of thepresent invention, both natural graphite and artificial graphite can beused. In particular, flaky natural graphite is preferable. Among the rawgraphite, the one having a maximum diffraction peak in the X-raydiffraction pattern by the CuKα line which appears at a diffractionangle (2θ, θ: Bragg angle) in a range from 26.2 degrees to 26.5 degrees,that is, where an interval between two graphite layers is equal to orless than 0.34 nm, is desirable. As a result, a graphite powdercontaining a small amount of the rhombohedral structure can be obtainedfrom the high crystalline raw material.

[0024] As for the pulverizing apparatus for crushing the raw graphite toa particle diameter equal to or less than 100 μm, a jet mill isdesirable. The reason is that the amorphous carbon is generated lesswith the jet mill than in the case when another pulverizing apparatus isused.

[0025] The pulverized raw graphite (raw powder) contains graphite havinga rhombohedral structure by approximately 30% as previously described.Then, in accordance with the present manufacturing method 1, theexisting fraction of the rhombohedral structure is decreased by thefollowing heat treatment.

[0026] The heat treatment is performed to at least 900° C. under aninert gas atmosphere. As for the inert gas, nitrogen gas, argon gas, andthe like is used. The inert gas atmosphere can also be maintained bycovering the raw powder with cokes to seal it from the atmosphere.

[0027] The heat treatment is the most important process in the presentinvention for transforming the rhombohedral structure to a hexagonalstructure. It is necessary to perform the heat treatment afterpulverization of the raw graphite (more preferably, at the last stage ofthe graphite powder manufacturing process of the present invention).

[0028] If the heat treatment is performed before the pulverization ofthe graphite and subsequently the graphite is pulverized, graphitepowder containing a rhombohedral structure in a quantity as small aspossible, which is the object of the present invention, can not beobtained. The graphite powder containing the rhombohedral structuregraphite in a quantity as small as possible can be obtained only byemploying the heat treatment after the pulverizing process (morepreferably, at the last stage of the graphite powder manufacturingprocess of the present invention) as the present invention proposes.

[0029] The raw graphite powder contains Al, Ca, Fe, and particularly alarge amount of Si, as impurities. The impurities can be eliminated byheating and sublimating the materials to at least 2700° C. Therefore,the heating temperature in the heat treatment is preferably at least2700° C. in order to perform a purification treatment concurrently.

[0030] (Manufacturing Method 2)

[0031] The raw graphite and the pulverizing process is the same as theabove manufacturing method 1.

[0032] The graphite powder of the present invention can be obtained bytreating the graphite powder obtained by the pulverizing process with anacidic solution containing at least one compound selected from a groupconsisting of sulfuric acid, nitric acid, perchloric acid, phosphoricacid, and fluoric acid, and subsequently washing with water,neutralizing, and drying. During the treatment, a compound is formedwith anions in the above acidic solution and the graphite, and therhombohedral structure graphite is eliminated by the formation of thecompound. The anions from the acidic solution in the compound areeliminated from the compound during the washing, the neutralizing, andthe drying, and the graphite powder relating to the present inventioncan be obtained.

[0033] The crystalline structure of the graphite powder of the presentinvention obtained by the above manufacturing methods 1 and 2 wasanalyzed by X-ray diffraction. The ratio of P₁ and P₂, (P₂/P₁), was lessthan 0.92, and the half band width of P₄ was less than 0.45 degrees. Theratio of P₁ and P₃, (P₃/P₁), was less than 0.75.

[0034] By substituting the above observed data for the equations 1 and2, the fact that the existing fraction of the rhombohedral structure hasbeen decreased to less than 20% and the existing fraction of thehexagonal structure has been increased at least 80% was confirmed.Simultaneously, the content of Si was confirmed to be less than 10 ppmfrom the result of impurity analysis.

[0035] Then, an electrode was prepared using the graphite powder of thepresent invention as an active material, and the lithiumintercalation-deintercalation capacity was studied. As a result, thelithium intercalation-deintercalation capacity of the graphite powder ofthe present invention was 320-360 mAh/g per unit weight of the activematerial, and the capacity was significantly improved in comparison withthe capacity of the conventional graphite material (200-250 mAh/g).Furthermore, it was found that the preferable existing fraction of therhombohedral structure was equal to or less than 10%, because the lessthe existing fraction of the rhombohedral structure in the graphitepowder of the present invention is, the more will the capacity beincreased. Where the fraction of the rhombohedral structure is 10% orless, and from Equations 91) and 92) herein, respectively, P₂/P₁ is 0.41or less and P₃/P₁ is 0.33 or less.

[0036] Accordingly, the rhombohedral structure is evidently acrystalline structure which hardly will intercalate or deintercalatelithium. Therefore, it is assumed that the high lithiumintercalation-deintercalation capacity of the graphite powder of thepresent invention is achieved by especially decreasing the existingfraction of the rhombohedral structure and increasing the existingfraction of the hexagonal structure.

[0037] The feature of the lithium secondary battery of the presentinvention is in using the graphite powder of the present invention asthe negative active material. The lithium secondary battery relating tothe present invention has a large load capacity, and a high energydensity can be realized.

[0038] As a result of an evaluation of the characteristics of thelithium secondary battery of the present invention, it was confirmedthat the lithium secondary battery of the present invention had asuperior performance in rapid charging and discharging characteristics,and a decreasing ratio of the capacity was improved at least 30% incomparison with the conventional lithium battery under a same rapidcharging and discharging condition. The reason for the improvement canbe assumed to relate to the fact that the reversibility for the lithiumintercalation-deintercalation reaction of the graphite of the presentinvention is improved in comparison with the conventional carbonmaterial by decreasing the existing fraction of the rhombohedralstructure and eliminating the influence of the impurities, such as Si.

[0039] As the positive active material for the lithium secondary batteryof the present invention, materials such as Li_(x)CoO₂, Li_(x)Nio₂,Li_(x)Mn₂O₄, (where, X is in a range 0-1) and the like are desirablebecause a high discharge voltage of at least 3.5 V can be obtained, andthe reversibility of the charging and discharging of the positiveelectrode itself is superior.

[0040] As for the electrolytic solution, a mixed solvent composed ofethylene carbonate mixed with any one selected from a group consistingof dimethoxyethane, diethylcarbonate, dimethylcarbonate,methylethylcarbonate, γ-butylolactone, methyl propionate, and ethylpropionate, and at least one of the electrolytes selected from a groupconsisting of salts containing lithium such as LiClO₄, LiPF₆, LiBF₄,LiCF₃SO₃, and the like are used. It is desirable to adjust the lithiumconcentration in a range 0.5-2 mol/l, because the electric conductivityof the electrolytic solution will be favorably large.

BRIEF DESCRIPTION OF DRAWINGS

[0041]FIG. 1 is a graph which indicates an X-ray diffraction pattern ofconventional graphite;

[0042]FIG. 2 is a graph which indicates an X-ray diffraction pattern ofgraphite powder relating to the embodiment 1 of the present invention(heat treatment temperature: 900° C.);

[0043]FIG. 3 is a graph which indicates an X-ray diffraction pattern ofthe graphite powder relating to the embodiment 1 of the presentinvention (heat treatment temperature: 2850° C.);

[0044]FIG. 4 is a graph which indicates an X-ray diffraction pattern ofthe graphite powder prepared in the comparative example 1;

[0045]FIG. 5 is a graph which indicates an X-ray diffraction pattern ofthe graphite powder relating to the embodiment 2 of the presentinvention;

[0046]FIG. 6 is a schematic cross section of the battery used in theembodiment 3 and the comparative example 2;

[0047]FIG. 7 is a graph indicating a relationship between the electrodepotential and the lithium intercalation-deintercalation capacity;

[0048]FIG. 8 is a partial cross section of the lithium secondary batteryprepared in the embodiment 5 of the present invention;

[0049]FIG. 9 is a graph indicating a relationship between the dischargecapacity and the number of repetitions of the charge and the dischargecycles;

[0050]FIG. 10 is a graph indicating a relationship between the dischargecapacity and the charging and discharging current;

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0051] Referring to the drawings, embodiments of the present inventionwill be explained hereinafter.

[0052] Embodiment 1

[0053] Flaky natural graphite which was produced from Madagascar wasused as the raw material, and the raw material was pulverized to form apowder, of which the particle diameter was equal to or less than 46 μm,using a jet mill. The powder was sieved to obtain a raw material powder.The average diameter of the raw material powder was 8.0 μm.Subsequently, the raw material powder was processed with a heattreatment by heating at 900° C. or 2850° C. for ten days under anitrogen atmosphere, and the graphite powder of the present inventionwas obtained.

[0054] The crystalline structures of the graphite powder of the presentinvention and the raw material powder were analyzed by an X-raydiffraction method using an apparatus RU-200 made by Rigaku Denki, andthe impurity content was analyzed by inductively coupled plasmaspectrometry (ICP) using an apparatus P-5200 made by Hitachi.

[0055] The X-ray diffraction patterns of the graphite powder of thepresent invention, which have been observed under a condition of X-raytube voltage of 40 kV, X-ray tube current of 150 mA, and X-ray source ofCuKα line, are shown in FIGS. 2 and 3. FIG. 2 is the pattern obtained bythe heat treatment at 900° C., and FIG. 3 is the pattern obtained by theheat treatment at 2850° C. The X-ray diffraction patterns of thegraphite powder of the present invention in both FIG. 2 and FIG. 3indicate that the peaks at diffraction angles of 43.3 degrees and 46.0degrees, both of which belong with the rhombohedral structure, aredecreased by either of the above heat treatments.

[0056] The amount of Si contained in the graphite powder of the presentinvention as an impurity was 1140 ppm when the heating temperature was900° C., and 27 ppm when the heating temperature was 2850° C. Therefore,it is revealed that a highly purified graphite powder, of which Si iseliminated, can be obtained by heat treatment at a high temperature atleast 2700° C., by which Si can be eliminated.

COMPARATIVE EXAMPLE 1

[0057] In order to provide a comparison with the embodiment of thepresent invention, non-pulverized raw graphite was heated at 2850° C.,and subsequently pulverized to obtain graphite powder. The X-ray patternof the graphite powder obtained by the above process is shown in FIG. 4.It is apparent from FIG. 4 that the peaks at diffraction angles of 43.3degrees and 46.0 degrees, both of which belong to the rhombohedralstructure, are not decreased. That means that the rhombohedral structurecan not be eliminated by the above process.

[0058] Embodiment 2

[0059] In accordance with the embodiment 2, raw graphite was pulverizedby a jet mill to less than 100 μm in particle diameter. Then, thegraphite powder was immersed into a mixed acid comprised of sulfuricacid and nitric acid for a whole day. Subsequently, washing withdistilled water and neutralization with a dilute aqueous solution ofsodium hydroxide were performed. The graphite powder obtained by theabove process was dried at 120° C. to obtain the graphite powder of thepresent invention. The X-ray pattern of the graphite powder obtained bythe above process is shown in FIG. 5. The peaks at diffraction angles of43.3 degrees and 46.0 degrees, both of which belong to the rhombohedralstructure, are decreased. Accordingly, it was found that therhombohedral structure was eliminated by the above process.

[0060] Embodiment 3

[0061] In accordance with the embodiment 3, a carbon electrode wasprepared using the graphite powder of the present invention as anelectrode active material, and the lithium intercalation-deintercalationcapacity, in other words, the load capacity of the negative electrode inthe lithium secondary battery, was studied with the electrode.

[0062] Mixed agents slurry were prepared by mixing 90% by weight intotal solid of the graphite powder of the present invention prepared inthe embodiment 1, 10% by weight of polyvinylidene fluoride (PVDF) as abinder, and N-methyl-2-pyrolidone, of which the heating temperatureswere 900° C. and 2850° C., respectively. The mixed agents slurry wasapplied on a plane of a sheet of copper foil of 10 μm thickness, anddried in a vacuum at 120° C. for one hour. After the vacuum drying, anelectrode was fabricated by roller pressing, of which the thickness wasin a range of 85-90 μm. The average amount of the applied mixed agentsper unit area was 10 mg/cm². The electrode was prepared by cutting thecopper foil with the applied mixed agents into a sheet of 10 mm sheet X10 mm.

[0063]FIG. 6 is a schematic cross section of a battery used for studyingthe lithium intercalation-deintercalation capacity of the presentelectrode. The battery has a structure, wherein a working electrodecurrent collector 30, the electrode of the present invention 31, whichis a working electrode, a separator 32, a lithium metal element 33,which is a counter electrode, and a counter electrode current collector34 are piled and inserted into a battery vessel 35, and a battery lid 36is screwed on for fixing. A reference electrode made of lithium metal 37is attached to the battery. As for the electrolytic solution, a mixedsolvent of ethylene carbonate and diethylcarbonate at a ratio of 1:1 involume and lithium hexafluorophosphate were used with a lithiumconcentration of 1 mol/l.

[0064] The intercalation-deintercalation of lithium was repeated byapplying a constant current between the working electrode and thecounter electrode, and the capacity was determined. The terminatedpotentials of the intercalation and the deintercalation of the workingelectrode were set as 0 V and 0.5 V, respectively.

COMPARATIVE EXAMPLE 2

[0065] In order to provide a comparison with the embodiment of thepresent invention, a carbon electrode was prepared with the graphitepowder obtained in the comparative example 1 by the same method as theembodiment 3, and the load capacity (the amount of lithiumintercalation-deintercalation) was determined. The same study wasperformed on the electrode prepared with the conventional graphitepowder (the same powder as the raw powder in the embodiment 1).

[0066] A result of comparison on the lithiumintercalation-deintercalation behavior of the electrode in theembodiment 3 (the present invention) with the electrode in thecomparative example 2 (prior art) and the electrode prepared with theconventional graphite powder will be explained hereinafter. FIG. 7 is agraph indicating a relationship between the lithiumintercalation-deintercalation capacity and the electrode potential atthe fifth cycle, wherein the capacity becomes stable, after repeatingthe intercalation-deintercalation of lithium. In FIG. 7, the curve 40indicates the potential variation of the electrode prepared with thegraphite powder, of which the heating temperature during the heattreatment was 900° C., in the embodiment 3. The curve 41 indicates thepotential variation of the electrode prepared with the graphite powder,of which the heating temperature during the heat treatment is 2850° C.,in the embodiment 3. The curve 42 indicates the potential variation ofthe electrode prepared with the conventional graphite powder, and thecurve 43 indicates the potential variation of the electrode preparedwith the graphite powder which has been prepared in the comparativeexample 1 by the reversely ordered processes. The intercalation capacityand the deintercalation capacity for lithium in both the cases of usingthe conventional graphite in the comparative example 2 (the curve 42)and the graphite in the comparative example 1 (the curve 43) were lessthan 250 mAh/g per unit weight of the active materials. On the contrary,in the case of the embodiment 3 (the curves 40, 41), wherein thegraphite powder prepared in the embodiment 1 was used as the activematerial, both the intercalation capacity and the deintercalationcapacity for lithium were more than 300 mAh/g per unit weight of theactive materials. That means that a large load capacity was obtained byusing the graphite powder having a small existing fraction of therhombohedral structure relating to the present invention. Furthermore,the case (the curve 41) using the graphite powder highly purified byheating up to 2850° C. indicates the largest values in both theintercalation capacity and the deintercalation capacity for lithium inFIG. 7.

[0067] Embodiment 4

[0068] The embodiment 4 was performed in order to confirm the influenceof treating time in the heat treatment of the present invention. In theembodiment 4, the graphite powder of the present invention was obtainedin substantially the same manner as the embodiment 1 (under a nitrogenatmosphere, the raw powder was heated at 2850° C.). However, thetreating time of the heat treatment was varied in a range from 0 hoursto 30 days.

[0069] The existing fraction of the rhombohedral structure wasdetermined from the peak intensity in X-ray diffraction patterns.Furthermore, as in the embodiment 3, the electrodes were prepared withthe obtained graphite powders, and the intercalation-deintercalationreactions of lithium were repeatedly performed. The result on thelithium intercalation-deintercalation capacity at the fifth cycle isshown in Table 1. TABLE 1 The existing fraction of Lithium Lithium theintercalation deintercalation rhombohedral capacity capacity Heatingtime structure (%) (mAh/g) (mAh/g)  0 hours 27.3 249 235  4 hours 18.2332 320 10 hours 14.6 345 325  1 day 13.8 343 334  3 days 11.3 355 338 5 days 9.7 368 351 10 days 7.1 365 360 30 days 3.9 366 361

[0070] In accordance with the above result, it is apparent that thesmaller the existing fraction of the rhombohedral structure is, the morewill the lithium intercalation-deintercalation capacity be increased. Inparticular, an existing fraction equal to or less than 10% is desirable.

[0071] Embodiment 5

[0072] The present embodiment uses a cylindrical lithium secondarybattery. A fundamental structure of the secondary battery is shown inFIG. 8. In FIG. 8, the member identified by the reference numeral 50 isa positive electrode. Similarly, a negative electrode 51, a separator52, a positive electrode tab 53, a negative electrode tab 54, a positiveelectrode lid 55, a battery vessel 56, and a gasket 57 are shown. Thelithium secondary battery shown in FIG. 8 was prepared by the followingsteps. Mixed positive electrode agents slurry was prepared by mixing 88%by weight in total solid of LiCoO₂ as an active material for thepositive electrode, 7% by weight of acetylene black as a conductiveagent, 5% by weight of polyvinylidene fluoride (PVDF) as a binder, andN-methyl-2-pyrolidone.

[0073] Similarly, mixed negative electrode agents slurry was prepared bymixing 90% by weight in total solid of the graphite powder of thepresent invention as an active material for the negative electrode, 10%by weight of polyvinylidene fluoride (PVDF) as a binder, andN-methyl-2-pyrolidone.

[0074] The mixed positive electrode agents slurry was applied onto bothplanes of a sheet of aluminum foil of 25 μm thickness, and dried in avacuum at 120° C. for one hour. After the vacuum drying, an electrode of195 μm thickness was fabricated by roller pressing. The average amountof the applied mixed agents per unit area was 55 mg/cm². The positiveelectrode was prepared by cutting the aluminum foil with the appliedmixed agents into a sheet 40 mm in width and 285 mm in length. However,portions of 10 mm in length from both ends of the positive electrodewere not applied with the mixed agents for the positive electrode, thealuminum foil was bared, and one of the bared portions was welded to thepositive electrode tab by ultrasonic bonding.

[0075] The mixed negative electrode agents slurry was applied onto bothplanes of a sheet of copper foil of 10 μm thickness, and dried in avacuum at 120° C. for one hour. After the vacuum drying, an electrode of175 μm thickness was fabricated by roller pressing. The average amountof the applied mixed agents per unit area was 25 mg/cm². The negativeelectrode was prepared by cutting the copper foil with the applied mixedagents into a sheet 40 mm in width and 290 mm in length. However, aswith the positive electrode, portions 10 mm in length from both ends ofthe negative electrode were not applied with the mixed agents for thenegative electrode, the copper foil was bared, and one of the baredportions was welded to the negative electrode tab by ultrasonic bonding.

[0076] A fine pored film made of polypropylene of 25 μm thickness and 44mm in width was used as a separator. The positive electrode, theseparator, the negative electrode, and the separator were piled in theorder described above, and the pile was rolled to form a bundle ofelectrodes. The bundle was contained in a battery vessel, the negativeelectrode tab was welded to the bottom of the battery vessel, and adrawn portion for caulking the positive electrode lid was fabricated. Anelectrolytic solution prepared by adding lithium hexafluorophosphate by1 mol/l into a mixed solvent containing ethylene carbonate anddiethylcarbonate by 1:1 in volume was filled in the battery vessel, thepositive electrode tab was welded to the positive electrode lid, and thepositive electrode lid was caulked to the battery vessel to form thebattery.

[0077] Using the battery which had been prepared by the above steps,charging and discharging were repeated under a condition in which thecharging and discharging current was 300 mA, and respective ones of theterminated potentials of the charging and the discharging were 4.2 V and2.8 V. Furthermore, the charging and the discharging currents werevaried in a range from 300 mA to 900 mA, and rapid charging and rapiddischarging were performed.

COMPARATIVE EXAMPLE 3

[0078] In order to provide a comparison with the present invention, alithium secondary battery was manufactured by the same method asembodiment 5 using the conventional graphite powder (the raw powder forthe graphite powder of the present invention), and the batterycharacteristics were determined in the same way as embodiment 5.

[0079] The result of comparison of the characteristics of the lithiumsecondary battery of the embodiment 5 (the present invention) and thecomparative example 3 (prior art) will be explained hereinafter.

[0080]FIG. 9 indicates a variation in discharge capacity of the lithiumsecondary battery when the charging and discharging of the battery wererepeated. The curve 60 indicates the discharge capacity of theembodiment 5. The curve 61 indicates the discharge capacity of thecomparative example 3. In the embodiment 5, the maximum dischargecapacity was 683 mAh, and the ratio in the discharge capacity after 200cycles to the maximum capacity was 86%. While, in the comparativeexample 3, the maximum discharge capacity was 492 mAh, and the ratio inthe discharge capacity after 200 cycles to the maximum capacity was 63%.

[0081]FIG. 10 indicates a relationship between the charging current anddischarging current and the discharge capacity when rapid charging andrapid discharging were performed. The curve 70 indicates the dischargecapacity of the embodiment 5. The curve 71 indicates the dischargecapacity of the comparative example 3. With a charging current anddischarging current of 900 mA, the discharge capacity of the embodiment5 was 573 mAh, while the discharge capacity of the comparative example 3was 256 mAh. The ratio of decrease of the discharge capacity in therespective ones of the present cases to the discharge capacity in thecase of the charging and discharging current of 300 mAh/g were 16% and48%, respectively. Therefore, by using the graphite powder of thepresent invention as the active material for the negative electrode, theratio of decrease of the capacity was improved by at least 30%, and itbecame apparent that the lithium secondary battery relating to thepresent invention had an excellent characteristics for rapid chargingand discharging.

[0082] Embodiment 6

[0083] Mixed positive electrode agents slurry was prepared using LiMn₂O₄as a positive electrode active material, and the positive electrode wasprepared by applying the mixed positive electrode agents slurry ontoboth planes of a sheet of aluminum foil in the same manner as embodiment5. The average amount of the applied mixed agents per unit area was 65mg/cm², and the electrode thickness after fabrication by roller pressingwas 230 μm. The positive electrode was prepared by cutting the aluminumfoil with the applied mixed agents into a sheet 40 mm in width and 240mm in length. However, portions 10 mm in length from both ends of thepositive electrode were not applied with the mixed agents for thepositive electrode. The negative electrode was the same as the negativeelectrode prepared in the embodiment 5. Then, the lithium secondarybattery of the present embodiment was prepared by the same method as theembodiment 5, such as by forming an electrodes bundle, inserting theelectrodes bundle into a vessel, welding a bottom of the vessel, addingan electrolytic solution, caulking a positive electrode lid, and others.

[0084] Using the battery, charging and discharging were repeated under acondition of a charging and discharging current of 300 mA, and theterminated potential of the charging and discharging of 4.2 V and 2.8 V,respectively. As a result, the maximum discharge capacity was 581 mAh,and the ratio of the discharge capacity after repeating the charging anddischarging reactions 200 cycles to the maximum discharge capacity was84%. The above result indicates that the charging and dischargingcharacteristics of the present embodiment are superior to thecomparative example 3.

[0085] A lithium secondary battery which has a high energy density andexcellent charging and discharging characteristics can be obtained byusing the graphite powder, which is superior in reversibility of theintercalation-deintercalation reaction of lithium, of which the maximumparticle size is less than 100 μm, wherein the existing fraction of therhombohedral structure in the crystalline structure is less than 20%, asthe active material for the negative electrode of the battery.

What is claimed is:
 1. A non-aqueous secondary battery, comprising: apositive electrode, a negative electrode, and an electrolytic solution,which is charged or discharged by repeating a reaction of intercalatingand deintercalating ions at said positive electrode and said negativeelectrode, respectively, wherein said negative electrode comprisesgraphite powder which has a particle size equal to or smaller than 100μm and which has an intensity ratio (P₂/P₁) equal to or less than 0.92,wherein P₁ is a diffraction peak of hexagonal crystal structure whichappears in a range of the diffraction angle from 41.7 degrees to lessthan 42.7 degrees and P₂ is a diffraction peak of rhombohedral crystalstructure which appears in a range of the diffraction angle from 42.7degrees to 43.7 degrees in a X-ray diffraction pattern with the CuKαline.
 2. A non-aqueous secondary battery as claimed in claim 1, whereinsaid graphite has an intensity ratio (P₂/P₁) equal to or less than 0.92,wherein P₁ is a diffraction peak which appears in a range of thediffraction angle from 41.7 degrees to 42.7 degrees and P₂ is adiffraction peak which appears in a range of the diffraction angle from42.7 degrees to 43.7 degrees in a X-ray diffraction pattern with theCuKα line.
 3. A non-aqueous secondary battery as claimed in claim 1,wherein said graphite has an intensity ratio (P₃/P₁) equal to or lessthan 0.75, wherein P₁ is a diffraction peak which appears in a range ofthe diffraction angle front 41.7 degrees to 42.7 degrees and P₃ is adiffraction peak which appears in a range of the diffraction angle from45.3 degrees to 46.6 degrees in a x-ray diffraction pattern with theCuKα line.
 4. A non-aqueous secondary battery, comprising: a positiveelectrode, a negative electrode, and an electrolytic solution, which ischarged or discharged by repeating a reaction of intercalating anddeintercalating ions at said positive electrode and said negativeelectrode, respectively, wherein said negative electrode comprisesgraphite powder which has a particle size equal to or smaller than 100μm and which has an intensity ratio (P₃/P₁) equal to or less than 0.75,wherein P₁ is a diffraction peak of hexagonal crystal structure whichappears in a range of the diffraction angle from 41.7 degrees to lessthan 42.7 degrees and P₃ is a diffraction peak of rhombohedral crystalstructure which appears in a range of the diffraction angle from 45.3degrees to 46.6 degrees in a X-ray diffraction pattern with the CuKαline.
 5. A non-aqueous secondary battery as claimed in claim 4, whereinsaid graphite has an intensity ratio (P₂/P₁) equal to or less than 0.92,wherein P₁ is a diffraction peak which appears in a range of thediffraction angle from 41.1 degrees to 42.7 degrees and P₂ is adiffraction peak which appears in a range of the diffraction angle from42.7 degrees to 43.7 degrees in a X-ray diffraction pattern with theCuKα line.
 6. A non-aqueous secondary battery as claimed in claim 4,wherein said graphite has an intensity ratio (P₃/P₁) equal to or lessthan 0.75, wherein P₁ is a diffraction peak which appears in a range ofthe diffraction angle from 41.7 degrees to 42.7 degrees and P₃ is adiffraction peak which appears in a range of the diffraction angle from45.3 degrees to 46.6 degrees in a X-ray diffraction pattern with theCuKα line.
 7. Electrodes for a non-aqueous secondary battery,comprising: a positive electrode, and a negative electrode, wherein saidnegative electrode comprises graphite powder which has a particle sizeequal to or smaller than 100 μm and which has an intensity ratio (P₂/P₁)equal to or less than 0.92, wherein P₁ is a diffraction peak ofhexagonal crystal structure which appears in a range of the diffractionangle from 41.7 degrees to less than 42.7 degrees and P₂ is adiffraction peak of rhombohedral crystal structure which appears in arange of the diffraction angle from 42.7 degrees to 43.7 degrees in aX-ray diffraction pattern with the CuKα line.
 8. Electrodes for anon-aqueous secondary battery as claimed in claim 7, wherein saidgraphite has an intensity ratio (P₂/P₁) equal to or less than 0.92,wherein P₁ is a diffraction peak which appears in a range of thediffraction angle from 41.7 degrees to 42.7 degrees and P₂ is adiffraction peak which appears in a range of the diffraction angle from42.7 degrees to 43.7 degrees in a X-ray diffraction pattern with theCuKα line.
 9. Electrodes for a non-aqueous secondary battery as claimedin claim 7, wherein said graphite has an intensity ratio (P₃/P₁) equalto or less than 0.75, wherein P₁ is a diffraction peak which appears ina range of the diffraction angle from 41.7 degrees to 42.7 degrees andP₃ is a diffraction peak which appears in a range of the diffractionangle from 45.3 degrees to 46.6 degrees in a X-ray diffraction patternwith the CuKα line.
 10. Electrodes for a non-aqueous secondary battery,comprising: a positive electrode, and a negative electrode, wherein saidnegative electrode comprises graphite powder which has a particle sizeequal to or smaller than 100 μm and which has an intensity ratio (P₃/P₃)equal to or less than 0.75, wherein P₁ is a diffraction peak ofhexagonal crystal structure which appears in a range of the diffractionangle from 41.7 degrees to less than 42.7 degrees and P₃ is adiffraction peak of rhombohedral crystal structure which appears in arange of the diffraction angle from 45.3 degrees to 46.6 degrees in aX-ray diffraction pattern with the CuKα line.
 11. Electrodes for anon-aqueous secondary battery, comprising: a positive electrode, and anegative electrode, wherein said negative electrode comprises graphitepowder which has an intensity ratio (P₂/P₁) equal to or less than 0.92,wherein P₁ is a diffraction peak of hexagonal crystal structure whichappears in a range of the diffraction angle from 41.7 degrees to lessthan 42.7 degrees and P₂ is a diffraction peak of rhombohedral crystalstructure which appears in a range of the diffraction angle from 42.7degrees to 43.7 degrees in a X-ray diffraction pattern with the CuKαline.
 12. Electrodes for a non-aqueous secondary battery, comprising: apositive electrode, and a negative electrode, wherein said negativeelectrode comprises graphite powder which has an intensity ratio (P₃/P₁)equal to or less than 0.75, wherein P₁ is a diffraction peak ofhexagonal crystal structure which appears in a range of the diffractionangle from 41.7 degrees to less than 42.7 degrees and P₃ is adiffraction peak of rhombohedral crystal structure which appears in arange of the diffraction angle from 45.3 degrees to 46.6 degrees in aX-ray diffraction pattern with the CuKα line.